Production of cement clinker

ABSTRACT

To permit the utilization of high-sulfur content solid fuels in the production of cement clinker in rotary kilns, the solid fuel is gasified and the gasified fuel is contacted with a fraction of the raw material used for the production of the cement clinker to desulfurize the gasified fuel, the desulfurized gasified fuel being delivered to a calcination stage and/or the rotary kiln for furnishing calories thereto.

The present invention relates to improvements in a process andinstallation for the production of cement clinker from dry raw material,such as finely divided cement raw meal.

It is known in the production of cement clinker to preheat the rawmaterial in heat exchange with combustion gases exhausted from therotary kiln and, optionally, to pass the preheated raw material througha calcination stage and to furnish sufficient calories to thecalcination stage for calcining the preheated raw material before it isdelivered to the rotary kiln.

For economic reasons, it has been proposed to utilize such poor solidfuels as oil shale, industrial or agricultural wastes, and the like incement production plants, as elsewhere. When such fuels containsubstantial proportions of sulfur, their use in cement production posesproblems because they tend to cause encrustations on the walls of plantcomponents, which leads to operating failures and necessitates more orless frequent plant shut-downs. In an effort to remedy this condition,it has been proposed to eject a fraction of the gases exhausted from therotary kiln to the atmosphere before they are delivered to the heatexchange means wherein they serve to preheat the raw material. Thisconventional solution to the problem has the disadvantage of reducingthe thermal efficiency of the plant. It has the further disadvantage ofrequiring costly gas purification devices to avoid air pollution.

It is a primary object of this invention to permit a more economical useof poor solid, sulfur-containing fuels in the production of cementclinker in processes and installations of the indicated type.

According to one aspect of the invention, this and other objects areaccomplished with a process wherein at least a portion of the caloriesrequired for producing the cement clinker in the rotary kiln isfurnished by a solid fuel, the solid fuel is gasified to produce thecalories, the gasified fuel is contacted with a fraction of thepreheated raw material until it has been desulfurized, the reactionproducts of the desulfurization are separated from the gasified fuel,and the desulfurized gasified fuel is delivered to the kiln for burning.Optionally and preferably, the preheated raw material is passed througha calcination stage and a sufficient amount of the calories is furnishedto the calcination stage for calcining the preheated raw material, thedesulfurized gasified fuel being delivered to the calcination stage.

In accordance with another aspect of the present invention, the aboveand other objects are accomplished with an installation which comprisesa rotary kiln for producing the cement clinker and having an inlet forthe raw material and a gas exhaust. A multi-stage heat exchange means isconnected to the gas exhaust for preheating the raw material in contactwith gas exhausted from the kiln and a means for burning a solid fuelproduces a combustible gas feeding calories to the kiln. A conduitconnects the solid fuel burning means to the kiln, the conduit having avertical portion with a lower end connected to the solid fuel burningmeans whereby the combustible gas passes in an upward draft through thevertical portion. A raw material distributor means divides the rawmaterial into two fractions, the distributor means having an outletconnected to the lower end of the vertical conduit portion fordelivering one of the raw material fractions into the vertical conduitportion whereby the one raw material fraction is suspended in the upwarddraft of the combustible gas before it is delivered to the kiln.Optionally and preferably, a calcination stage is connected to the heatexchange means for calcining the preheated raw material before it isdelivered to the rotary kiln and the vertical conduit portion isconnected to the calcination stage whereby the combustible gas furnishescalories to the calcination stage for calcining the preheated rawmaterial.

In the production of cement clinker according to this invention, thecalcium carbonate, which is a constituent of the dry raw material, isdecomposed in contact with the gasified fuel into hot carbonic gases andlime which reacts with any sulfur compounds in the fuel to produce acalcium sulfide and this can be separated from the gasified fuel beforeit is delivered to the calcination stage and/or the kiln. In thismanner, the fuel used in the cement clinker production is desulfurizedwithout thermal losses. Another advantage of this procedure resides inthe fact that the decomposition of calcium carbonate is an endothermicreaction which cools the gas and, therefore, makes it possible to gasifythe fuel at a high temperature without necessitating the provision ofhigh refractory linings in the conduit connecting the solid fuel burningmeans to the calcination stage or kiln.

If the heat exchange means used for preheating the dry raw material is amulti-stage heat exchanger, particularly in case the heat exchangerstages are constituted by cyclones, the fraction of the raw materialused for the desulfurization of the gasified fuel may be removed at theoutlet of the last heat exchanger stage or from an intermediate stage.It could also be removed at the inlet of the heat exchanger.

The above and other objects, advantages and features of the presentinvention will become more apparent from the following detaileddescription of a now preferred embodiment thereof, taken in conjunctionwith the single FIGURE of the accompanying schematic drawingillustrating an installation for producing cement clinker in accordancewith this invention.

Referring now to the drawing, the installation is shown to compriserotary tubular kiln 10 which has an upstream end with respect to thecirculation of the dry raw material which is introduced into kiln 10through this end for conversion to cement clinker. Casing 37 defining asmoke chamber is affixed to kiln 10 at the upstream end and receives hotflue gases from the kiln, the gases being exhausted from the kiln toflow in a first direction. The kiln is heated by burner 38 mounted atthe opposite, downstream end and the kiln is slightly inclined tofacilitate the flow of the material along the kiln from the upstream tothe downstream end where the cement clinker is discharged into cooler16, all in an entirely conventional manner.

The illustrated heat exchange means for preheating the dry raw materialin heat exchange with combustion gases exhausted from the rotary kilnbefore it is delivered into the kiln is constituted by two groups ofmulti-stage heat exchangers. In the preferred embodiment illustratedherein, the installation comprises calcination stage 12 connected to theheat exchange means for calcining the preheated raw material before itis delivered to rotary kiln 10, and one group of the multi-stage heatexchangers is associated with the kiln while the other group isassociated with the calcination stage. The stages of the illustratedheat exchangers are constituted by cyclones.

As shown, the first group of multi-stage heat exchangers is connected tothe gas exhaust of kiln 10 and comprises cyclones 5, 4, 3 and twincyclone 2 connected in series by flues 20, 22 and 24, flue 18 connectingthe kiln gas exhaust to cyclone 5 and the exhaust gases flowing throughthese flues and series-connected cyclones in a first direction indicatedby arrows showing the updraft of the gases flowing through the heatexchange means. Flue 25 connects twin cyclone 2 to fan 26 which producesthe gas flow in this first direction. The outlets of the cyclones areconnected in series with the inlets of the preceding cyclones so as toproduce a counter-current flow of raw material through the heatexchanger, the dry raw material being suspended in the updraft of theexhaust gases and flowing therethrough in a second direction. As shown,the outlets of twin cyclone 2 are connected by conduit 21 to flue 22leading to preceding cyclone 3, the outlet of cyclone 3 is connected byconduit 19 to flue 20 leading to preceding cyclone 4, and the outlet ofcyclone 4 is connected by conduit 17 to flue 18 leading to precedingcyclone 5. Conduit 27 connects the outlet of cyclone 5 to flue 28 whichconnects clinker cooler 16 to calcination stage 12 to conduct cool airfrom the cooler to the calcination stage.

The second group of multi-stage heat exchangers comprises cyclones 9, 8,7, 6 and twin cyclone 1 connected in series by flues 48, 30, 32 and 34,the inlet of cyclone 9 being connected to calcination stage 12 by flue11. Means 14 for burning a solid fuel to produce a combustible gasfeeding calories to calcination stage 12 and kiln 10 is connected byflue 40 to the calcination stage, the gasified fuel flowing in a firstdirection indicated by arrows through the flues connecting thecalcination stage and cyclone stages of the heat exchanger. The outletof twin cyclone 1 is connected by flue 45 to fan 36 to produce the gasflow in this direction in an updraft. The dry raw material is fed to theinstallation at A into flue 34 where it is carried into twin cyclone 1.The outlets of this twin cyclone are connected by conduits 23 and 31respectively to conduit 24 in the first group of heat exchangers todeliver a first portion of the raw material to this group of heatexchangers for preheating and to conduit 32 in the second group of heatexchangers for delivering a second portion of the raw material to thesecond group of heat exchangers for preheating. The cyclones of thesecond group of heat exchangers are series-connected, conduit 31 leadinginto flue 32, conduit 29 connecting the outlet of cyclone 6 to flue 30,conduit 43 connecting the outlet of cyclone 7 to conduit 48 and theoutlet of cyclone 9 being connected to the inlet of kiln 10 by conduit13.

According to this invention, conduit 40 connecting solid fuel burnermeans 14 to calcination stage 12 and thus to kiln 10 has verticalportion 40a with a lower end connected to the solid fuel burner meanswhereby the combustible gas generated in means 14 passes in an upwarddraft through the vertical portion and furnishes calories to calcinationstage 12 for calcining the preheated raw material. Preferably, as shown,cyclone 42 is connected to the upper end of vertical conduit portion 40afor separating the material suspended in the upward draft of thecombustible material from the gas. The preferred embodiment of the fuelburning means illustrated herein comprises moving grid 15 supporting afluidized bed of the solid fuel. The grid is so inclined, or has atleast a downstream end so inclined, that the downstream end of themobile grid is situated at a level at least as high as, or higher than,the upper level of the fluidized bed.

The outlet of cyclone 8 in the second group of multi-stage heatexchangers is connected in accordance with the invention with rawmaterial distributor means 44 for dividing the preheated and calcinedraw material into two fractions. The raw material distributor means hasan outlet connected to the lower end of vertical conduit portion 40a fordelivering one fraction of the raw material into the vertical conduitportion whereby this raw material fraction is suspended in the upwarddraft of the combustible gas coming from burner means 14 before it isdelivered to the calcination stage and the kiln. Distributor means 44has another outlet connected to flue 28 for delivering another rawmaterial fraction to this flue wherein the air coming from cooler 16carries the other raw material fraction to calcination stage 12 forcalcining. The fraction of the raw material suspended in the gasifiedfuel in vertical conduit portion 40a causes the gasified fuel to bedesulfurized and the reaction products of the desulfurization areseparated from the gasified fuel in cyclone 42 before the fuel is usedin the cement clinker production, the desulfurized gasified fuel beingdelivered to the calcination stage and the kiln through the hereinabovedescribed circuit of flues in which the material flows countercurrently.

Control 46 for raw material distributor 44 is responsive to thetemperature of the gasified fuel in conduit 40 to regulate the rawmaterial fractions delivered from the respective outlets of distributor44 to flue 28 and vertical conduit portion 40a, respectively. It hasbeen found that, normally, the fraction of the preheated raw materialdelivered to vertical conduit portion 40a for desulfurization of thegasified fuel constitutes about 2% to 10%, by weight, of the total rawmaterial preheated in heat exchange with the combustion gases exhaustedfrom the kiln 10 and from the calcination stage 12.

According to the preferred feature of the present invention, control 46will be so set that the temperature of the gasified fuel downstream fromthe point of injection of the preheated raw material fraction rangesbetween about 800° C. and 950° C. The gasification of the solid fuel ispreferably effected in a fluidized bed of fuel and ashes at atemperature permitting the agglomeration of the ash particles, and theseagglomerated particles are removed by the moving grid.

The operation of the hereinabove-described installation will partiallybe obvious from the illustrated structure and will now be described indetail.

The raw material introduced at A is carried by the upward draft of thedesulfurized gasified fuel circulating through flues 11, 48, 30, 32 and34 into twin cyclone 1 where the material is divided into twosubstantially equal portions, one of the portions being delivered byconduit 23 into the first group of heat exchangers 2, 3, 4 and 5 whilethe other portion is introduced by conduit 31 into the second group ofheat exchangers 6, 7, 8 and 9. The one raw material portion is preheatedby the countercurrent flow of gases exhausted from kiln 10 and thepreselected material is delivered from cyclone 5 through conduit 27 intoflue 28 where the cool air from cooler 16 entrains the preheated rawmaterial portion to calcination stage 12.

The other raw material portion is preheated by the countercurrent flowof the gases flowing from the calcination stage through flues 11, 48,30, 32 and 34, and the preheated material is delivered from cyclone 8 todistributor 44 where it is divided into two fractions of considerablydiffering proportions. The much larger fraction is delivered to flue 28where it mixes with the raw material portion coming from the first groupof heat exchangers and flows to the calcination stage. The much smallerraw material fraction used for the desulfurization of the gasified fuelin conduit 40 is injected into the conduit at the lower end of verticalconduit portion 40a. The distributor is controlled by gage 46 which, asindicated by broken lines, is connected to distributor 44 as well as tovertical conduit portion 40a so that the control is responsive to thetemperature of the gases in this conduit portion.

The desulfurized gasified fuel coming from cyclone 42 burns incalcination stage 12 with the air coming from cooler 28 to furnish thecalories required for the calcination of the raw material. The calcinedmaterial leves the calcination stage through flue 11 to enter cyclone 9where the calcined material is separated from the gases and introducedinto rotary kiln 10 through conduit 13 for conversion into cementclinker. The discharged clinker is then cooled by air introduced intocooler 16 and the cooling air is then used as secondary air in the kilnfor burning the fuel introduced by burner 38 and in the calcinationstage for burning the gasified fuel coming from solid fuel burning means14 where air injected under grid 15 maintains the solid fuel in afluidized state and assures its partial combustion. The air flow is keptsubstantially below the flow theoretically necessary for combustion andis so controlled that the temperature in the fluidized fuel bed ismaintained at an elevated level sufficient to permit agglomeration ofthe ash particles forming the fuel and the agglomerated particles areremoved by deposition on the grid and its removal by the moving grid.

The gases produced in solid fuel burner means 14 have a temperature ofthe order of about 1,100° C. to 1,200° C. The fraction of the rawmaterial introduced by distributor means 44 into vertical conduitportion 40a has the effect of lowering this temperature to about 900° C.and assures the desulfurization thereof according to the followingreaction:

    CaCO.sub.3 →CaO+CO.sub.2

    CaO+H.sub.2 s→CaS+H.sub.2 O

These reactions are enhanced by a gas temperature in the range of about800° C. to 950° C. and a fine granulation of the raw material,preferably to average a grain size of less than about 100 microns. Underthese conditions, the desulfurization reactions proceed very rapidly.

The resultant reaction products as well as any particles of ashesentrained by the gasified fuel are separated from the desulfurizedgasified fuel in cyclone 42.

If desired, a first cyclone for separating the ashes from the gasifiedfuel could be placed at the outlet of solid fuel burning means 14 andthe separated fuel particles may be returned from this cyclone to means14 while the raw material used for desulfurization of the gasified fuelis then injected in the stream of gaseous fuel downstream of said firstcyclone and separated therefrom in a second cyclone mounted fartherdownstream in conduit 40. The calcium sulfide formed in thedesulfurization reactions and separated from the gasified fuel may beeconomically used to produce gypsum added to the crushed clinker for themanufacture of cement.

The fraction of the raw material fed from cyclone 8 to vertical conduitportion 40a is so regulated by control 46 that the temperature of thegasified fuel downstream from the point of injection of thedesulfurizing raw material is maintained at a value equal to the nominalvalue, for example of the order of 900° C.

While the desulfurizing fraction of raw material is illustrated as beingremoved from cyclone 8 of the second group of heat exchangers, it wouldbe possible to remove it from any cyclone of the two groups of heatexchangers. If desired, it would even be possible to use cold rawmaterial for this purpose, in which case distributor means 44 would bemounted at A upstream of the heat exchange means.

The described and illustrated process and installation for producingcement clinker makes it possible to desulfurize gasified fuels used tofurnish calories to the kiln and/or the calcination stage much morethoroughly and at a much lower cost than has been possible inconventional procedures. Furthermore, if the solid fuel burner means isoperated at a temperature at which the ashes are agglomerated, most ofthe ash particles will be easily and dependably removed by the movinggrid in the burner means, rather than being entrained by the upwarddraft of the gasified fuel and only a small amount of ashes remaining tobe separated in cyclone 42. In this way, fuel producing a great amountof ashes may be used, such as oil shale or bituminous fuels, withoutharming the quality of the cement clinker.

While the present invention has been described and illustrated inconnection with a now preferred embodiment thereof, it will beunderstood that many variations and modifications may occur to thoseskilled in the art, particularly after benefitting from the presentteaching, without departing from the spirit and scope of this invention,as defined in the appended claims.

What is claimed is:
 1. A process of producing cement clinker from a dryraw material in a rotary kiln, which comprises the steps of(a)preheating the raw material in heat exchange with combustion gasesexhausted from the rotary kiln, (b) furnishing at least a portion of thecalories required for producing the cement clinker by a solid fuel, (c)gasifying the solid fuel to produce said calories, (d) contacting thegasified fuel with a fraction of the preheated raw material until it hasbeen desulfurized, (e) separating the reaction products of thedesulfurization from the gasified fuel, and (f) burning the desulfurizedgasified fuel to deliver at least a portion of the calories required forproducing the clinker.
 2. The cement clinker producing process of claim1, further comprising the steps of passing the preheated raw materialthrough a calcination stage and furnishing a sufficient amount of saidcalories to the calcination stage for calcining the preheated rawmaterial, and delivering the desulfurized gasified fuel to thecalcination stage.
 3. The cement clinker producing process of claim 1 or2, wherein the gasified fuel is circulated in an upward draft and thepreheated raw material is injected into, and suspended in, the upwarddraft of the gasified fuel for desulfurizing the gasified fuel.
 4. Thecement clinker producing process of claim 1 or 2, wherein the fractionof the preheated raw material constitutes about 2% to 10%, by weight, ofthe raw material preheated in heat exchange with the combustion gasesexhausted from the kiln and eventually from the calcination stage. 5.The cement clinker producing process of claim 3 or 4, wherein thetemperature of the gasified fuel downstream from the point of injectionof the preheated raw material ranges between about 800° C. and 950° C.6. The cement clinker producing process of claim 5, wherein the flow ofthe preheated raw material injected into the upward draft of thegasified fuel is controlled for maintaining the temperature of thegasified fuel downstream from the point of injection equal to apredetermined range.